The field of the invention is rechargeable batteries.
Secondary (rechargeable) batteries are utilized in a great variety of applications, including large mechanical devices such as electric vehicles, mobile or immobile battery backup systems, all manner of communications and electronic devices, and even individual electronic chips.
Depending on the particular application involved, it is generally advantageous to employ either a high power battery or a high energy battery. In some military applications, for example, such as where a projectile is propelled by magnetic forces, it is desirable to have a high power battery which delivers a large amount of energy over a very short period of time. In many other applications, such as industrial high torque valves, communication via satellite cellular phones, ignition systems, and medical defibrillation, it is also desirable to employ high power batteries. In contrast, high energy batteries are generally favored for portable consumer electronic devices such as lighting, portable computers and ground-based cellular telephones. In such applications energy is drawn from the battery relatively slowly over many hours, or even days.
There is a tradeoff between high power and high energy characteristics of batteries, and it is exceedingly difficult to produce high energy batteries which also have very high power. Among other things, a high cycle life which is generally desirable in high energy batteries becomes more difficult to achieve as the power increases. Such power/energy tradeoff is related in some measure to battery chemistry. A typical lead-acid battery, for example, may have a peak sustained power (over at least 10 seconds) of 500 W/Kg, and a total energy of 35 W-Hour/Kg at full discharge, where the weight in kg represents the total weight of the battery, less packing. In contrast, a typical lithium ion battery may have a peak sustained power of 250 W/kg, and a total energy of 125 W-Hour/Kg. Typical values for these and other chemistries using prior art technologies in batteries having a cycle life of at least 250 cycles are estimated in Table 1 below.
In addition to battery chemistry, the power and energy characteristics of a battery relate to morphology of the electrodes. Modern electrodes generally comprise fine spherical particles (approx. 20-50 microns) compacted together about a rod or sheet shaped conductive substrate. As such, there are really two different morphologies to be considered, the morphology of the individual particles (microscopic morphology), and the morphology of the entire electrode (macroscopic morphology).
The microscopic morphology greatly influences both power and energy capacity. In general, smaller particles provide a greater surface area and therefore a greater power, while larger particles provide a smaller surface area and less power. This is only true up to a point, however, because at some point finer particles pack less closely together, and tend to lose electrical contact with each other and the current collector. This, in turn, tends to yield less efficient utilization on a mass basis. In addition, smaller particles tend to be more difficult to pack onto the conductive substrate. This tends to yield less total mass of active electrode material than electrodes made with larger particles, which thereby tends to reduce total energy capacity.
The macroscopic morphology also has a considerable influence on both power and energy capacity. Thin film or interdigitated electrodes, for example, provide relatively large surface areas and therefore relatively high power, while simple tubular electrodes with relatively small surface areas tend to provide relatively low power. With respect to energy capacity, the key factor is not so much surface area, but the total mass of active electrode material which is in sufficient proximity to the electrolyte to receive mobile ions.
There are many known methods for coating a conductive substrate with an active electrode material. Typical methods include spray coating or spray deposition, and techniques along these lines are described in U.S. Pat. No. 5,721,067 to Jacobs et al. (February, 1998), U.S. Pat. No. 4,649,061 to Rangachar (1987), and U.S. Pat. No. 5,589,300 to Fateau et al. (1996). In general, spraying technologies include the use of ultrasonic or air spraying. Alternative coating methods such as roll coating, casting, electrospray, thermal spray, ultrasonic spray, vapor deposition, powder coating, etc. are also known.
One consequence of the known methods of fabricating electrodes is a trade-off between power and energy capacity. The known methods tend to deposit particles in layers on the conductive substrate, and since there is generally only point contact between adjacent particles in such layers, migration of ions from the electrolyte to the conductive substrate is slowed as the number of layers is increased. In this manner, attempts to increase power by reducing the particle size tends to increase the number of particle layers, and thereby reduce the energy capacity. As a result, a typical P/E ratio for rechargeable batteries is about 3 hrxe2x88x921. Those skilled in the art will recognize that it is traditional in the field to omit the units, hrxe2x88x921, when describing power/energy ratios, and that tradition is generally followed hereinafter.
A trade-off between power and energy capacity may be satisfactory in many applications, but it may be undesirable in other applications. In power tools, hybrid vehicles, and cellular communications, for example, it is desirable to have both high power, defined herein to be at least 800 W/kg sustainable over a 10 second period, and high energy capacity, defined herein to be more than about 5 W-hour/kg at full discharge. In still other applications, it may be desirable to not only have high power and high energy capacity, but also to have a high power to energy capacity ratio, i.e., a high P/E ratio of 10, 20, 30 or more. Batteries that can fulfill all of these requirements, while maintaining relatively high cycle life are unknown. Thus, there is still a need for improved batteries, and methods of fabricating electrodes for such batteries.
In one aspect of the invention, methods and apparatus are provided in which rechargeable batteries having cycle life to deep discharge of at least 250 cycles provide power of at least 600 W/kg and energy of at least 5 W-hr/kg. In preferred embodiments such batteries provide power of at least 800 W/kg and at least 7 W-hr/kg, or at least 700 W/kg and at least 15 W-hr/kg. In still more preferred embodiments such batteries provide power of at least 1000 W/kg and at least 9 W-hr/kg, and at least 700 W-hr/kg and at least 20 W-hr/kg.
In another aspect of the invention, methods and apparatus are provided in which rechargeable batteries having cycle life to deep discharge of at least 250 cycles have a power to energy (P/E) ratio of at least 10. For some applications, more preferred embodiments have a P/E ratio of at least 20, still more preferred embodiments have a P/E ratio of at least 50, and still more preferred embodiments have a P/E ratio of at least 100.
In another aspect of the invention, electrodes in batteries are fabricated by providing electrodes with high aspect ratio subparts. In one class of embodiments the subparts comprise active material particles coated onto a conductive substrate. In another class of embodiments, the subparts comprise microplates extending from a conductive substrate.
These and other advantages and attainments of the present inventive matter will become apparent to those skilled in the art upon a reading of the following detailed description, when taken in conjunction with the drawings, wherein like components are referenced using like numerals.